This invention relates to air valve assemblies usable in gas turbine engines. Specifically, this invention relates to air valve assemblies utilized in premixers of the type having a rotatable-type air valve actuated by a shaft penetrating a pressure housing.
It is known from U.S. Pat. No. 5,377,483, for example, to utilize fuel/air premixers to provide a closely controlled fuel/air mixture in single stage combustors for gas turbine engines. The close control afforded by such combustor premixer systems results in controlled combustion temperatures and low levels of NOx, CO, and unburnt hydrocarbon emissions, as well as low losses due to the overall control of the compressed air inventory.
It is also known from U.S. Pat. No. 5,377,483 that the close control of the fuel/air ratio can be advantageously achieved through the use of an air valve. Preferably, the air valve should be located in the relatively "cool" part of the combustor system outside of the combustion chamber liner. This location for the premixer and the resident air valve can minimize or mitigate problems associated with differential thermal expansion and temperature-related stress and fatigue problems traditionally associated with a valve-type apparatus mounted directly on or within the combustion chamber liner.
However, the temperatures at the "cool" location of such premixers can still be elevated as a consequence of an adiabatic temperature rise of the compressed air being controlled, which can range to upwards of 600.degree. C. Problems can still exist, therefore, in terms of the construction and operation of movable valve elements such as butterfly-type valve plates and their associated actuating shafts. Moreover, if the actuating shaft requires penetration of the main pressure vessel housing for the engine, adequate sealing must be provided to prevent the leakage of hot compressed air through the penetration past the shaft, both from a thermal efficiency standpoint and also for safety purposes. Still further, the actuator shaft arrangement must be capable of delivering precise angular control to the valve plate in order to achieve the desired close control of the fuel/air ratio of the mixture.